As the experience of studying Mowgli children who were raised by animals of different biological species (wolves, monkeys, puma, red Asian wolves (dholi), bears, etc.) shows us, they were never able to master normal human speech after that , as they were discovered in nature, caught and returned to human society, even though they got to animals, as a rule, at the age of 2-5 years, that is, already having some level of verbal communication. These children had a good command of the communication systems of the animal species that raised them, both sound and so-called. "body language" (a system of carrying information for other individuals of body positions). After entering human society, such children with great difficulty managed to instill speech skills at the level of people with oligophrenia in the degree of imbecility, although they were not mentally retarded and in situations not related to the use of speech showed a good level of intelligence. That is, human speech is wholly and completely a product of the social interaction of a child and speech carriers - his parents, relatives and the surrounding society.
None. By the way, for this it is not necessary to really deprive the child of the environment. There have been cases when middle preschool children who cannot speak were taken from disadvantaged families.